Hello,

I am using codesys Modbus TCP client (4.4.0.0) to communicate with a python modbus server (package pyModbusTCP).

I first start my python server on the distant machine.
After a fresh codesys compilation, a plc connection/transfer and a PLC run, the modbus connection is OK. Every time in this context the connection is created correctly.

When I shut down the server, the modbus connection falls, that is normal.
When I restart the python server, impossible to recreate the modbus connexion. With the client autoreconnection, I see on my server that the client tries to connect but unsuccesfully. I have the following message :

DEBUG:pyModbusTCP.server:accept new connection from ClientInfo(address='192.168.1.20', port=33476)
DEBUG:pyModbusTCP.server:Exception during request handling: NetworkError('recv return null')

I tried to stop the codesys client and to restart it with these commands :

Modbus_TCP_Client.xStop := True;  // Or False
Modbus_TCP_Client.Enable := True;  // Or False
Modbus_TCP_Server.Enable := True;  // Or False

I tried to confirm the error manually to force reconnection with:

Modbus_TCP_Server_Motors.xConfirmError := TRUE;

I tried also this command to STOP/RESET/START client and server (codesys side) but nothing happens :

status_client := Modbus_TCP_Client.SetCommunicationState(eRequestedState := DED.DEVICE_TRANSITION_STATE.STOP);  // .RESET & .START

The answer of this function when executed is "status_client = NOT_SUPPORTED". Is it normal ?

It seems that is a socket problem, but I do not understand if it is on the client or server side.
I tried a modbus simulator called "ananas.exe" and the result is the same. Impossible to get a modbus client reconnection.

What is different at the first client connection and at a reconnection attempt ?

Thanks for your help

Problem solved

The client modbus cycle time was linked to a task at 500ms. Whereas the channel modbus was at 200ms. Certainly problem of packages synchronisation.

I let the client modbus cycle time to be handled by itself (See attachement) --> "Use the bus cyle time parameters" (traduce from french)
See attachement